U.S. Pat. No. 4,856,993 (hereinafter the '993 patent), which is assigned to the assignee of the current application, describes a variable resistance pressure sensing array and a circuit for scanning such array. Such arrays may, for example, be used as a contact sensor for dental occlusion, to detect foot pressure in a shoe, to detect pressure distribution on a gasket, and in numerous other applications where information on pressure or force distribution over a defined area is required. While the device of this patent represented a substantial step forward in the art at the time the application was filed, and the technology disclosed in this patent is still being utilized for many applications, improvements are possible in the performance of this circuit in areas such as response time, precision of the outputs, and sensitivity control. It is also desirable to reduce potential cross-talk between electrodes. Among other things, this would eliminate the need for the variable resistance layers 74 shown in FIG. 2b of the '993 patent, the elimination of this extra layer of pressure responsive ink or other pressure sensitive material resulting in a significant reduction in the fabrication cost of the array and also resulting in enhanced resolution.
More particularly, each sensor point where the electrodes intersect in a sensing array of the type shown in the '993 patent also has a trace capacitance which is charged during sampling of the point. If such capacitance must discharge through the interelectrode pressure sensitive resistance between successive applications of a test voltage or other signal to the electrodes, the time for the capacitances to fully discharge or for the circuit to settle between each application of test voltage can be up to one millisecond. For large arrays with perhaps hundreds of electrodes in each direction, this can result in substantial lost time in completing a single scan of the entire array, and can make the time for completing such a scan unacceptably long. It would be preferable if this time could be reduced by at least an order of magnitude, and preferably to something in the ten to twenty microsecond range.
Further, trace resistances in the array, such as those for the electrodes and the multiplexers, cause IR drops for test voltages applied to the array. Since the current being drawn varies with the load on the array, the voltage drop across the various trace resistances of the device also varies with load. This means that the test voltage applied to the sensing points varies with the pressure applied to the sensor point and with the number of sensor points having pressure applied thereto, resulting in errors which may be in the ten to twenty-five percent range in sensed pressures. A need therefore exists for an improved circuit which more accurately compensates the test voltage applied to the array for variations in load so as to enhance the accuracy of readings.
Another problem is in selecting pressure resolution for the array. Currently, different arrays having different inks or other variable resistance material are required in order to achieve different resolutions. For applications where the resolution or pressure range are substantially different for different areas of the array, either an array which is not optimum for any area must be utilized, or an expensive customized array must be fabricated. However, the customized array must be carefully aligned to achieve the desired results, and may, if somewhat misaligned, cause more harm than good. It would therefore be preferable if resolution variations could be achieved electronically, permitting either a single array to be constructed which could be used for all applications, or at least a very limited number of different arrays to cover the entire resolution range. Such array could also be calibrated for different ranges/resolutions in different areas.
Finally, in many applications there are outputs which are not of a sufficient level to be relevant for a particular application, or outputs may be redundant. Since the conversion of analog outputs to digital for processing and the processing of such outputs take substantial time, the speed of operation for the device can also be enhanced if such operations are only performed for points of concern and not for points which are either not of concern (for example being below a predetermined pressure level) or redundant. In particular, in many applications, pressure is only being applied to a few sensor points on the array and substantial processing time can be saved if processing is performed only for such points, rather than for all points on the array.